This invention relates to a cyclic process for the removal of NO.sub.x and SO.sub.x from a gas stream using a water-soluble, polymeric chelate of a polyvalent metal and membrane separation means to recover and recycle the chelate for reuse.
The combustion of fossil fuels to generate electrical power commonly produces flue gases containing levels of NO.sub.x (nitrogen oxides) and SO.sub.x (sulfur oxides) which are environmentally harmful. NO.sub.x includes NO and NO.sub.2 ; SO.sub.x includes SO.sub.2 and SO.sub.3. Typical flue gases may contain between about 10 and about 2000 ppmv NO.sub.x and between about 100 ppmv and 5.0 mole percent SO.sub.x, with SO.sub.2 constituting the primary contaminant. The emission levels of NO.sub.x and SO.sub.x are subject to both federal and state air quality control requirements. Methods of reducing NO.sub.x and SO.sub.x levels in flue gases include cleaning fuel to remove sulfur or switching to fuel containing less sulfur, controlling combustion conditions such as fuel/air ratio and temperature to minimize NO.sub.x formation, and flue gas clean-up through scrubbing.
Common methods of removing SO.sub.x from flue gases involve scrubbing with alkalis of calcium, magnesium, or sodium. Such methods generally are ineffective for removing NO.sub.x from flue gases. NO.sub.x in the presence of SO.sub.x may be removed from flue gases by absorbing the NO.sub.x in sulfite solutions containing iron salts to produce imidodisulfonates with subsequent hydrolysis to ammonium sulfate. NO.sub.x may also be removed by treating flue gases with a solution containing a water-soluble monomeric or polymeric iron chelate. Problems associated with these processes include inefficient separation of excess water, excess build-up of monomeric reaction products, loss of expensive chelates, and use of expensive water-soluble alkali.
What is needed is an inexpensive process for removing NO.sub.x and SO.sub.x from flue gases which removes excess water and monomeric reaction products, preserves chelates within the process, and utilizes inexpensive lime as the primary source of alkali within the process.